Lý thuyết giá trị cực đoan - Hiển thị: Bình thường đến Gumbel

Tối đa của $X_1,\dots,X_n. \sim$ iid Standardnormals hội tụ vào Phân phối Gumbel tiêu chuẩn theo Lý thuyết giá trị cực đoan .

Làm thế nào chúng ta có thể chỉ ra điều đó?

Chúng ta có

$$P(\max X_i \leq x) = P(X_1 \leq x, \dots, X_n \leq x) = P(X_1 \leq x) \cdots P(X_n \leq x) = F(x)^n$$

We need to find/choose $a_n>0,b_n\in\mathbb{R}$ sequences of constants such that:

$$F\left(a_n x+b_n\right)^n\rightarrow^{n\rightarrow\infty} G(x) = e^{-\exp(-x)}$$

Can you solve it or find it in literature?

There are some examples pg.6/71, but not for the Normal case:

$$\Phi\left(a_n x+b_n\right)^n=\left(\frac{1}{\sqrt{2\pi}}\int_{-\infty}^{a_n x+b_n} e^{-\frac{y^2}{2}}dy\right)^n\rightarrow e^{-\exp(-x)}$$

An indirect way, is as follows:
For absolutely continuous distributions, Richard von Mises (in a 1936 paper "La distribution de la plus grande de n valeurs", which appears to have been reproduced -in English?- in a 1964 edition with selected papers of his), has provided the following sufficient condition for the maximum of a sample to converge to the standard Gumbel, $G(x)$:

Let $F(x)$ be the common distribution function of $n$ i.i.d. random variables, and $f(x)$ their common density. Then, if

$$\lim_{x\rightarrow F^{-1}(1)}\left (\frac d{dx}\frac {(1-F(x))}{f(x)}\right) =0 \Rightarrow X_{(n)} \xrightarrow{d} G(x)$$

Using the usual notation for the standard normal and calculating the derivative, we have

$$\frac d{dx}\frac {(1-\Phi(x))}{\phi(x)} = \frac {-\phi(x)^2-\phi'(x)(1-\Phi(x))}{\phi(x)^2} = \frac {-\phi'(x)}{\phi(x)}\frac {(1-\Phi(x))}{\phi(x)}-1$$

Note that $\frac {-\phi'(x)}{\phi(x)} =x$. Also, for the normal distribution, $F^{-1}(1) = \infty$. So we have to evaluate the limit

$$\lim_{x\rightarrow \infty}\left (x\frac {(1-\Phi(x))}{\phi(x)}-1\right)$$

But $\frac {(1-\Phi(x))}{\phi(x)}$ is Mill's ratio, and we know that the Mill's ratio for the standard normal tends to $1/x$ as $x$ grows. So

$$\lim_{x\rightarrow \infty}\left (x\frac {(1-\Phi(x))}{\phi(x)}-1\right) = x\frac {1}{x}-1= 0$$

and the sufficient condition is satisfied.

The associated series are given as

$$a_n = \frac 1{n\phi(b_n)},\;\;\; b_n = \Phi^{-1}(1-1/n)$$

ADDENDUM

This is from ch. 10.5 of the book H.A. David & H.N. Nagaraja (2003), "Order Statistics" (3d edition).

$\xi_a = F^{-1}(a)$. Also, the reference to de Haan is "Haan, L. D. (1976). Sample extremes: an elementary introduction. Statistica Neerlandica, 30(4), 161-172." But beware because some of the notation has different content in de Haan -for example in the book $f(t)$ is the probability density function, while in de Haan $f(t)$ means the function $w(t)$ of the book (i.e. Mill's ratio). Also, de Haan examines the sufficient condition already differentiated.

The question asks two things: (1) how to show that the maximum $X_{(n)}$ converges, in the sense that $(X_{(n)}-b_n)/a_n$ converges (in distribution) for suitably chosen sequences $(a_n)$ and $(b_n)$, to the Standard Gumbel distribution and (2) how to find such sequences.

The first is well-known and documented in the original papers on the Fisher-Tippett-Gnedenko theorem (FTG). The second appears to be more difficult; that is the issue addressed here.

Please note, to clarify some assertions appearing elsewhere in this thread, that

1. The maximum does not converge to anything: it diverges (albeit extremely slowly).

2. There appear to be different conventions concerning the Gumbel distribution. I will adopt the convention that the CDF of a reversed Gumbel distribution is, up to scale and location, given by $1-\exp(-\exp(x))$. A suitably standardized maximum of iid Normal variates converges to a reversed Gumbel distribution.

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Intuition

When the $X_i$ are iid with common distribution function $F$, the distribution of the maximum $X_{(n)}$ is

$$F_n(x) = \Pr(X_{(n)}\le x) = \Pr(X_1 \le x)\Pr(X_2 \le x) \cdots \Pr(X_n \le x) = F^n(x).$$

When the support of $F$ has no upper bound, as with a Normal distribution, the sequence of functions $F^n$ marches forever to the right without limit:

Partial graphs of $F_n$ for $n=1,2,2^2, 2^4, 2^8, 2^{16}$ are shown.

To study the shapes of these distributions, we can shift each one back to the left by some amount $b_n$ and rescale it by $a_n$ to make them comparable.

Each of the previous graphs has been shifted to place its median at $0$ and to make its interquartile range of unit length.

FTG asserts that sequences $(a_n)$ and $(b_n)$ can be chosen so that these distribution functions converge pointwise at every $x$ to some extreme value distribution, up to scale and location. When $F$ is a Normal distribution, the particular limiting extreme value distribution is a reversed Gumbel, up to location and scale.

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Solution

It is tempting to emulate the Central Limit Theorem by standardizing $F_n$ to have unit mean and unit variance. This is inappropriate, though, in part because FTG applies even to (continuous) distributions that have no first or second moments. Instead, use a percentile (such as the median) to determine the location and a difference of percentiles (such as the IQR) to determine the spread. (This general approach should succeed in finding $a_n$ and $b_n$ for any continuous distribution.)

For the standard Normal distribution, this turns out to be easy! Let $0 \lt q \lt 1$. A quantile of $F_n$ corresponding to $q$ is any value $x_q$ for which $F_n(x_q) = q$. Recalling the definition of $F_n(x) = F^n(x)$, the solution is

$$x_{q;n} = F^{-1}(q^{1/n}).$$

Therefore we may set

$$b_n = x_{1/2;n},\ a_n = x_{3/4;n} - x_{1/4;n};\ G_n(x) = F_n(a_n x + b_n).$$

Because, by construction, the median of $G_n$ is $0$ and its IQR is $1$, the median of the limiting value of $G_n$ (which is some version of a reversed Gumbel) must be $0$ and its IQR must be $1$. Let the scale parameter be $\beta$ and the location parameter be $\alpha$. Since the median is $\alpha + \beta \log\log(2)$ and the IQR is readily found to be $\beta(\log\log(4) - \log\log(4/3))$, the parameters must be

$$\alpha = \frac{\log\log 2}{\log\log(4/3) - \log\log(4)};\ \beta = \frac{1}{\log\log(4) - \log\log(4/3)}.$$

It is not necessary for $a_n$ and $b_n$ to be exactly these values: they need only approximate them, provided the limit of $G_n$ is still this reversed Gumbel distribution. Straightforward (but tedious) analysis for a standard normal $F$ indicates that the approximations

$$a_n^\prime = \frac{\log \left(\left(4 \log^2(2)\right)/\left(\log^2\left(\frac{4}{3}\right)\right)\right)}{2\sqrt{2\log (n)}},\ b_n^\prime = \sqrt{2\log (n)}-\frac{\log (\log (n))+\log \left(4 \pi \log ^2(2)\right)}{2 \sqrt{2\log (n)}}$$

will work fine (and are as simple as possible).

The light blue curves are partial graphs of $G_n$ for $n=2, 2^6, 2^{11}, 2^{16}$ using the approximate sequences $a_n^\prime$ and $b_n^\prime$. The dark red line graphs the reversed Gumbel distribution with parameters $\alpha$ and $\beta$. The convergence is clear (although the rate of convergence for negative $x$ is noticeably slower).

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References

B. V. Gnedenko, On The Limiting Distribution of the Maximum Term in a Random Series. In Kotz and Johnson, Breakthroughs in Statistics Volume I: Foundations and Basic Theory, Springer, 1992. Translated by Norman Johnson.